This invention relates generally to techniques for handling substrates processed in an automated fabrication system and, more particularly, to the alignment and positioning of rectangular substrates with respect to substrate cassettes and substrate loadlocks. Although there are many existing and proposed systems for moving circular semiconductor wafers in and out of processing chambers, the techniques employed for aligning and positioning circular wafers are, for the most part, inappropriate for the processing of rectangular substrates of glass or similar materials.
In a processing system with which the present invention is principally concerned, large rectangular glass substrates are presented to the system stacked in cassettes. Each cassette may hold a number of substrates in a vertical stack, with each substrate held from beneath by edge supports. In a cassette loading operation, a robot mechanism moves the substrates one by one from the cassettes to a loadlock, which has multiple vertical positions for the substrates. In the processing system, the substrates are removed from the loadlock through another door, and are subjected to a variety of processing steps, none of which is of concern to the present invention. Upon completion of processing, substrates are placed back in the loadlock, or perhaps in a different loadlock, and the robot mechanism moves the substrates from the loadlock back into cassettes.
The robot mechanism may be of any type that provides for movement of a workpiece support, referred to as an end effector, in three dimensions. In robot technology, it is common to "teach" a robot mechanism the position of an object or objects that are to be picked up and moved. The teaching mode of operation generally requires that the mechanism be moved under manual control to the desired position. In its automatic mode of operation, the mechanism moves repeatedly to the learned position where a workpiece is to be picked up and moved. The destination position of the workpiece is also taught to the robot mechanism. Two practical difficulties in the present system limit the accuracy of the substrate positions taught to the robot mechanism.
First, substrates are placed in a cassette within a deliberate angular and positional tolerance. In other words, the cassettes are made to provide a relatively loose fit for the substrates, both angularly and positionally. To make a cassette for a tighter fit with the substrates would inevitably result in scraping of the substrates on the cassette walls while placing the substrates in and removing them from the cassettes. Scraping or rubbing of the glass substrates is undesirable because it increases the probability of particulate contamination during processing. A second aspect of the positional uncertainty of the substrates results from a degree of uncertainty in the orientation and position of the cassettes themselves. The cassettes are placed manually on a work table, and positioned within locating ridges or other features of the table. Some degree of positional uncertainty is deliberately provided, to facilitate handling of the cassettes. The result of these positional uncertainties (of the cassettes with respect to the work table, and of the substrates with respect to the cassettes), is that a substrate position taught to the robot mechanism is only an approximation of the actual substrate position, and means must be provided to compensate for variations in the substrate position. In the past, alignment of rectangular substrates has been effected by means of movable edge guides that bodily move the substrates into conformance with a predicted position. The drawback of this approach is that it requires sliding contact with the substrate edges, which increases the likelihood of particulate contamination.
It will be appreciated from the foregoing that there is need for a better technique for aligning rectangular substrates with a robot arm that picks them up for movement to a loadlock or other processing chamber. Ideally, the technique should not require pushing, or otherwise repositioning the substrates, either in a cassette or when loaded onto the robot arm. The present invention addresses this need.